A man appointed as a conservationist caretaker of a pristine island scenic reserve set fire to a resort, causing nearly $300,000 of damage.

John Michael Scarlett, known as Will, pleaded guilty to a charge of arson at Auckland District Court and will be sentenced in February.

The 66-year-old resident of Great Barrier Island in Auckland's Hauraki Gulf was appointed caretaker of nearby Kaikoura Island by the Motu Kaikoura Trust. He did the job for several years before the trust opted not to renew his contract in July 2012.

Thirteen months later, Scarlett left his home in Port Fitzroy with a five-litre can of diesel and made the short journey across the water to the island.

In the early hours of October 7, 2013, he pulled mattresses and other material into the middle of the biggest building on the island and doused them in the fuel.

The 150sq m structure known as "The Lost Resort", which had been used as a recreation and kitchen area by hundreds of people, including the late yachting hero Sir Peter Blake, burned to the ground, leaving only ashes and blackened, corrugated metal.

Scarlett's actions showed some conflicting emotions. He removed boxes of personal items from the lodge before setting it alight and also watered an area around the building in an attempt to protect the native bush.

Regardless of that, trust chairman Harry Doig was livid with the "callous" actions.

"I feel incredibly angry and disappointed that something he has invested a lot of his time and effort in, he felt like he could just come and destroy, just with the striking of a match," Doig said.

"He calls himself a conservationist so I don't understand how he rationalises his actions."

Within hours of committing the crime, Scarlett returned to his home and phoned the local constable to tell her what he had done.

He was interviewed and made a full confession. "In explanation the defendant stated he was frustrated at not being listened to and that he wanted to make a statement," according to court documents.

But Doig said the trust had no idea what Scarlett's issue was. "I'm not aware at all that he felt aggrieved at the time . . . he was on a fixed-term contract that came to an end."

The charge of arson carries a maximum penalty of 14 years imprisonment but Doig said he was more interested in seeing Scarlett pay for his actions financially.

Although a reparation figure of $286,580 had been set, he believed the damage caused would cost much more to repair and planned to say that in a victim-impact statement to the court.

"I do think he should pay as much as he can afford to undo the harm that he's done but he's never going to undo the stress and anger that he's put people through," Doig said.

The trust chairman believed it was only good luck that surrounding buildings had not been set alight and the hillside above the lodge had not gone up.

Despite the setback, Doig vowed the trust would continue to pursue their goals of restoring Kaikoura Island's natural environment without pests and making it available to the public to enjoy its beauty.

The 564-hectare island surrounded by rugged cliffs was formerly owned by a US syndicate before being bought by the Crown in 2005.